Electric drills are well known for drilling holes in various materials and for driving screws, bolts and the like. Such drill motors commonly comprise a hand held or mounted motor, a chuck for holding a drill bit, power cord, and a trigger for applying power to the drill motor. The trigger is often also capable of varying the speed of the drill motor.
In conventional drilling, the drill bit is forced at a low speed, under pressure, toward the material to be drilled, thereby permitting the leading edge of the drill bit to bite into the material. Once the leading edge of the drill bit has begun cutting, the speed of the bit can be increased to a point where maximum drilling efficiency is achieved. At this speed the drill cuts into and removes material at the fastest rate. This results in a shaving action of the bit upon the material. However, after operating at this speed momentarily, the cutting edge of the bit tends to lose its bite and must be slowed so that the process can continue.
If the speed of the bit is not reduced after it loses its bite, then the cutting efficiency drops rapidly as the bit begins to chip, or drop in and out of the shaving action.
During optimum cutting, when the shaving action is achieved, a continuous curl of the drilled material forms at the drill bit. When chipping commences, the continuous curl breaks, thus signaling to the operator a drop in cutting efficiency. Upon noticing the broken curl, the operator may reduce the speed of the drill bit until shaving action resumes.
Oftentimes, however, the user merely notices the drop in cutting efficiency and responds by increasing the drilling pressure. It is a natural tendency to increase pressure when efficiency drops. The user attempts to force the bit into the material being drilled. The drill bit, having ceased cutting, initially increases in speed. Forcing the bit into the material heats up both the bit and material, thus reducing the speed of the bit. Typically the bit begins to smoke and may lose its temper. Once the temper is lost, the bit cannot continue to cut, but rather creates a glaze within the hole and also tends to case harden the material. Home users frequently do not use cutting oil or coolant when drilling, thus worsening the problem.
Typically, lower speeds are used for larger drills. This is necessary because on a large drill bit the outer portion of the cutting edge of the drill bit travels faster relative to the material being cut than on a small one. Thus, lower speeds are required for larger bits to prevent excessive heat build up along the outer portions of the cutting edge where friction is the greatest.
Not only does chipping reduce the cutting efficiency of the drill bit, it also leads to increased heat build up and even increases the possibility of binding the bit within the material.
Different materials require various bit speeds for drilling. For instance, steel can be cut at a higher speed than concrete. Each substance also has a different ideal cycle time for achieving optimal efficiency in the cutting process.
When drilling concrete, the drill bit speed must be kept below the point where the concrete becomes glazed. Once glazing occurs, a hard surface is formed which must be penetrated before further progress can be made. A lower speed and more rapid cycling is required with concrete so that a fresh bite can be maintained at the drill bit tip to insure against the build up of heat and glazing. Also, heavy pressure is required to maintain proper contact of the drill bit tip with the material being drilled to continue the shaving action.
When drilling a material having a thickness which is less than the length of the drill bit, the drill frequently breaks through the second side of the material and can bind and twist out of the hands of the operator or otherwise cause an accident. Even in cases where the drill bit does not bind, it may distort the hole as the drill speeds up and the bit is jammed forcibly into the completed hole. The operator will attempt to release the trigger and then back the drill out, however it is common for the drill to be angled slightly prior to releasing the trigger, thus resulting in a slightly off axis hole.
Typically, the operator attempts to slow the speed of the bit when it breaks through the second side of the material. This is a fairly difficult skill to master and requires that the operator pay close attention as drilling nears completion.
As the drill begins to cut through the second surface, a portion of the tip will start to protrude and the drill motor current will rise significantly due to the increased rate of feed as the tip protrudes. It would be desirable to sense the increase in drill motor current and reduce the speed of the drill in order to prevent the drill bit from speeding up and possibly binding and causing an accident.
Additionally, the drill bit can bind during the drilling process, likewise twisting the drill from the operators hands and possibly causing injury. In this case, drill motor current increases as the bit binds and the motor is prevented from turning. Thus, it would also be desirable to sense an increase in drill motor current and to rapidly cut power to the drill in order to prevent further binding and possible injury.
Power screw driving is gaining rapid acceptance because of the time and effort saved when compared to manual driving. Considerable time can be saved while expending relatively little effort by using power screw driving means.
Using a drill to screw bolts, screws or like fasteners often results in damaged fastener heads or broken fasteners. This is caused by over torquing of the fastener during the screwing process. A skilled operator can control the torque being applied to the fastener by the drill by varying the speed of the drill. However, this requires a considerable amount of skill and is much more difficult than the drilling process. Accidents resulting from the screwdriver bit slipping out of the fastener head are common. An automated method of controlling the torque would therefore be very desirable.
Another common problem typically encountered when using power screwing means is twisting off the head of the fastener. This typically occurs when driving fasteners into wood which is harder than usual, when a knot is encountered, or when the pilot hole is too small or nonexistent. Frequently, the fastener cannot be driven completely into the wood. The fastener binds and as additional torque is applied, the head is broken off. This also happens when tightening small bolts and machine screws.
While many operators are able to adequately duplicate the cycle required for efficient drilling and to adequately control the torque of a drill motor when tightening fasteners, the process requires a fair amount of skill to be performed efficiently. Even the most skilled operators rarely approach the efficiency theoretically achievable. Therefore, although the prior art has recognized to a limited extent the problem of obtaining optimum efficiency in the drilling and fastener tightening processes, the proposed solutions have to date been ineffective in providing a satisfactory remedy.